Copy process substituting copmpressible bit pattern for any unqualified data objects

ABSTRACT

A copy procedure detects qualified data objects in a body of source data, and copies the source data to a target storage unit except for unqualified data objects, which are replaced with a prescribed bit pattern. Following completion of the backup, a record is prepared indicating that all data objects exist in the specified target storage, regardless of whether each data object was replaced with a predetermined bit pattern rather than being physically written to the specified target storage. This process may, be repeated in order to perform data reclamation, effectively removing user files no longer qualifying for backup.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/752,240, filed on May 22, 2007, which is a continuation of U.S.patent application Ser. No. 11/057,737, filed on Feb. 14, 2005, whichissued as U.S. Pat. No. 7,275,075 on Sep. 25, 2007, which is acontinuation of U.S. patent application Ser. No. 10/055,635, filed onJan. 22, 2002, which issued as U.S. Pat. No. 6,941,328 on Sep. 6, 2005,and which patent applications and patents are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to digital data storage management. Moreparticularly, the invention concerns a copy procedure that distinguishesbetween qualified and unqualified data objects in a body of source data,and copies the source data to a target storage unit except forunqualified user files, which are replaced with a prescribedcompressible bit pattern. Regardless of whether data objects are copiedor replaced with the prescribed bit pattern, the copy process reportsthem as having been copied successfully.

2. Description of the Related Art

The electronic management of data is central in this information era.Scientists and engineers have provided the necessary infrastructure forwidespread public availability of an incredible volume of information.The Internet is one chief example. In addition, the high-technologyindustry is continually achieving faster and more diverse methods fortransmitting and receiving data. Some examples include satellitecommunications and the ever-increasing baud rates of commerciallyavailable computer modems.

With this information explosion, it is increasingly important for usersto have some means for storing and conveniently managing their data. Inthis respect, the development of electronic data storage systems is moreimportant than ever. And, engineers have squarely met the persistentchallenge of customer demand by providing speedier and more reliablestorage systems.

As an example, engineers at INTERNATIONAL BUSINESS MACHINES CORPORATION(IBM) have developed various flexible systems called “storage managementservers”, designed to store and manage data for remotely locatedclients. One example is the TIVOLI STORAGE MANAGER (TSM) product. Withthis product, a central server is coupled to multiple client platformsand one or more administrators. The server provides storage, backup,retrieval, and other management functions for the server's clients.

Although the TSM product includes some significant improvements overprior storage systems, IBM continually seeks to improve the efficiencyof this and other such systems. One area of possible focus is spaceutilization, namely, minimizing the amount of storage space required tostore data. To minimize the cost of disk, tape, and other storage media,customers wish to minimize the storage space that their data occupies.Customers also seek to minimize other storage assets, such as tapelibrary storage slots, etc. Although some useful approaches have beenproposed to address these concerns, IBM is nevertheless seeking bettersolutions to benefit its customers.

SUMMARY OF THE INVENTION

Broadly, the present invention concerns a copy procedure that detectsunqualified data objects in a body of source data, and copies the sourcedata to a target storage unit except for unqualified data objects, whichare replaced with a prescribed bit pattern. The invention detects andprocesses unqualified data objects whether they are “aggregated” or not.Aggregated data objects are data objects that have been concatenated forprocessing as a single unit to aid efficiency.

More specifically, a storage director initially reviews a body of sourcedata to determine whether its data objects are already present in targetstorage. Data objects already present in target storage need not becopied. As for data objects not present in target storage, the storagedirector selectively copies the data objects to target storage. Then,the storage director applies prescribed criteria (such asdifferentiating between predetermined “active” and “inactive” dataobject designations) to determine which of the data objects qualify forcopying, and which do not. Then, the storage director forms a “copy” ofthe source data on the target storage. In this copy operation, however,the storage director replaces each unqualified data object with apredetermined bit pattern. Responsive to completion of the copyoperation, the storage director prepares a record indicating that thedata object exists in target storage regardless of whether it wasphysically copied or replaced with the substitute bit pattern. Thestorage director may later repeat similar techniques to make a copy ofthe last copy, thereby performing a storage reclamation operation thatconsolidates storage to take advantage of any data objects that havebecome inactive after the first copy was made.

The foregoing features may be implemented in a number of differentforms. For example, the invention may be implemented to provide a methodof copying data. In another embodiment, the invention may be implementedto provide an apparatus such as a storage subsystem configured to copydata. In still another embodiment, the invention may be implemented toprovide a signal-bearing medium tangibly embodying a program ofmachine-readable instructions executable by a digital data processingapparatus to copy data as discussed herein. Another embodiment concernslogic circuitry having multiple interconnected electrically conductiveelements configured to copy data as disclosed herein.

The invention affords its users with a number of distinct advantages.For example, the copy technique disclosed herein may be used toimplement a backup operation that essentially limits backup to “active”files, and omits “inactive” files from backup. Rather than being copied,the inactive files are replaced with a predetermined substitute bitpattern. Moreover, this entire process may be repeated to implement areclamation process. By maintaining and using well organized metadata,active/inactive file status can be quickly determined with a minimum ofprocessing overhead. Importantly, storage space can be conserved byusing a substitute bit pattern that is highly compressible, for example,by hardware components that apply compression algorithms upon storage.Moreover, when substituting the prescribed bit pattern for any userfiles that are members of an aggregate file, the same length bit patternis used so that the bit pattern (when uncompressed) occupies the sameamount of storage as each respective substituted user file (whenuncompressed). Consequently, offsets of each data object within anaggregate file are retained, preserving the accuracy of the originalmetadata. The invention also provides a number of other advantages andbenefits, which should be apparent from the following description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of the hardware components andinterconnections of a storage management system in accordance with theinvention.

FIG. 1B is a block diagram showing the database component of FIG. lA ingreater detail.

FIG. 2 is a block diagram of a digital data processing machine inaccordance with the invention.

FIG. 3 shows an exemplary signal-bearing medium in accordance with theinvention.

FIG. 4A is a block diagram showing the subcomponents of an illustrativestorage hierarchy in accordance with the invention. FIG. 4B is a blockdiagram showing some contents of the storage hierarchy of

FIG. 4A in greater detail, and more particularly, the existence ofprimary storage pools and copy storage pools.

FIG. 5 is a block diagram showing the interrelationship of various userfiles and aggregate files.

FIG. 6 is a flowchart of an operational sequence for a copy processsubstituting a predetermined bit pattern for unqualified user files.

FIG. 7 is a flowchart of an operational sequence for restoring data fromone or more copy storage pools due to data of a primary storage poolbeing lost or inaccessible.

FIG. 8 is a flowchart of an operational sequence for restoring data fromthe storage hierarchy to a client station due to data becoming lost orinaccessible at that client station.

DETAILED DESCRIPTION

The nature, objects, and advantages of the invention will become moreapparent to those skilled in the art after considering the followingdetailed description in connection with the accompanying drawings.

Hardware Components & Interconnections Introduction

One aspect of the invention concerns a storage management system, whichmay be embodied by various hardware components and interconnections. Oneexample is shown by the storage management system 100 of FIG. 1A.Broadly, the system 100 includes a data storage subsystem 102, one ormore administrator stations 104, and one or more client stations 106.The subsystem 102 operates in response to directions of the clientstations 106, as well as the administrator stations 104. Theadministrator stations 104 are used by system administrators toconfigure, monitor, and repair the subsystem 102. Under direction of anend user, the client stations 106 use the subsystem 102 to store andmanage data on their behalf. More particularly, each client station 106creates and regards data in the form of “user files,” also called“client files.” In this regard, each client station 106 separatelyemploys the subsystem 102 to archive, back up, retrieve, and restore itsuser files. Optionally, each user file may be associated with a singleclient station 106, which is the source of that user file.

Client Stations

Each client station 106 may comprise a general purpose computer such asa file server, workstation, personal computer, etc. The client stations106 may comprise similar or different machines, running similar ordifferent operating systems. Some exemplary operating systems includeUNIX, OS/2, WINDOWS-NT, DOS, etc.

The client stations 106 are interconnected to the subsystem 102 by anetwork 116. The network 116 may comprise any desired connection,including one or more conductive wires or busses, fiber optic lines,data communication channels, wireless links, Internet, telephone lines,etc. In one example, a high speed communication channel such as a T3link is may be used, employing a network protocol such as TCP/IP.

Administrator Stations

The administrator stations 104 comprise electronic equipment for a humanor automated storage administrator to convey machine-readableinstructions to the subsystem 102. Thus, the stations 104 may compriseprocessor-equipped general purpose computers or “dumb” terminals,depending upon the specific application. The administrator stations 104may be coupled to the subsystem 102 directly or by one or more suitablenetworks (not shown).

Data Storage Subsystem: Subcomponents

In an exemplary embodiment, the data storage subsystem 102 may comprisea commercially available server such as an IBM TSM product. However,since other hardware arrangements may be used as well, a generalizedview of the subsystem 102 is discussed below.

The data storage subsystem 102 includes a storage director 108, having aconstruction as discussed in greater detail below. The storage director108 exchanges signals with the network 116 and the client stations 106via an interface 112, and likewise exchanges signals with theadministrator stations 104 via an interface 110. The interfaces 110/112may comprise any suitable device for communicating with the implementedembodiment of client station and administrator station. For example, theinterfaces 110/112 may comprise ETHERNET cards, small computer systeminterfaces (“SCSIs”), parallel data ports, serial data ports, telephonemodems, fiber optic links, wireless links, etc.

The storage director 108 is also coupled to a database 113 and a storagehierarchy 114. As discussed in greater detail below, the storagehierarchy 114 is used to store “managed files”. A managed file mayinclude an individual user file (stored as such), or multipleconstituent user files stored together as a single “aggregate” file.Although the term “file” is used for illustration, numerous other dataobjects may be utilized in place of a file, such as a table space,image, database, binary bit pattern, etc.

The subsystem's storage of user files protects these files from loss orcorruption on the client's machine, assists the clients by freeingstorage space at the client stations, and also provides moresophisticated management of client data. In this respect, operations ofthe storage hierarchy 114 include “backing up” files from the clientstations 106, backing up client stations' files contained in the storagehierarchy 114, “retrieving” stored files from the storage hierarchy 114for the client stations 106, and “restoring” files backed-up on thehierarchy 114.

The database 113 contains information (“metadata”) about the filescontained in the storage hierarchy 114. This information, for example,includes the addresses at which files are stored, variouscharacteristics of the stored data, certain client-specified datamanagement preferences, etc. The contents of the database 113 arediscussed in detail below.

More Detail: Exemplary Data Processing Apparatus

As mentioned above, the storage director 108 may be implemented invarious forms. As one example, the storage director 108 may comprise adigital data processing apparatus, as exemplified by the hardwarecomponents and interconnections of the digital data processing apparatus200 of FIG. 2.

The apparatus 200 includes a processor 202, such as a microprocessor,personal computer, workstation, or other processing machine, coupled toa storage 204. In the present example, the storage 204 includes afast-access storage 206, as well as nonvolatile storage 208. As oneexample, the fast-access storage 206 may comprise random access memory(“RAM”), and may be used to store the programming instructions executedby the processor 202. The nonvolatile storage 208 may comprise, forexample, battery backup RAM, EEPROM, one or more magnetic data storagedisks such as a “hard drive”, a tape drive, or any other suitablestorage device. The apparatus 200 also includes an input/output 210,such as a line, bus, cable, electromagnetic link, or other means for theprocessor 202 to exchange data with other hardware external to theapparatus 200.

Despite the specific foregoing description, ordinarily skilled artisans(having the benefit of this disclosure) will recognize that theapparatus discussed above may be implemented in a machine of differentconstruction, without departing from the scope of the invention. As aspecific example, one of the components 206, 208 may be eliminated;furthermore, the storage 204, 206, and/or 208 may be provided on-boardthe processor 202, or even provided externally to the apparatus 200.

More Detail: Storage Hierarchy

The storage hierarchy 114 may be implemented in storage media of variousnumber and characteristics, depending upon the clients' particularrequirements. To specifically illustrate one example, FIG. 4A depicts arepresentative storage hierarchy 400. The hierarchy 400 includesmultiple levels 402-410, where levels nearer the top of the figurerepresent incrementally higher levels of storage performance. The levels402-410 provide storage devices with a variety of features andperformance characteristics.

In this example, the first level 402 includes high-speed storagedevices, such as magnetic hard disk drives, writable optical disks, orother direct access storage devices (“DASDs”). The level 402 providesthe fastest data storage and retrieval time among the levels 402-410,albeit the most expensive. The second level 404 includes DASDs with lessdesirable performance characteristics than the level 402, but with lowerexpense. The third level 406 includes multiple optical disks and one ormore optical disk drives. The fourth and fifth levels 408-410 includeeven less expensive storage means, such as magnetic tape or anothersequential-access storage device.

The levels 408-410 may be especially suitable for inexpensive, long-termdata archival, whereas the levels 402-406 are appropriate forshort-term, fast-access data storage. As an example, one or more devicesin the level 402 and/or level 404 may even be implemented to provide adata storage cache.

Devices of the levels 402-410 may be co-located with the subsystem 102,remotely located, or a combination of both, depending upon the user'srequirements. Accordingly, storage devices of the hierarchy 400 may becoupled to the storage director 108 by a variety of means, such as oneor more conductive wires or busses, fiber optic lines, datacommunication channels, wireless links, Internet connections, telephonelines, SCSI connection, ESCON connection, etc.

Although not shown, the hierarchy 400 may be implemented with a singledevice type, and a corresponding single level. Ordinarily skilledartisans will recognize the “hierarchy” being used illustratively, sincethis disclosure contemplates but does not require a hierarchy of storagedevice performance.

More Detail: Storage & Copy Pools

Optionally, the storage hierarchy 400 may utilize storage poolsincluding primary storage pools and copy storage pools as shown by theexample of FIG. 4B. Primary copies of user data are stored in primarystorage pools such as 450-452 while backup copies of user data from theprimary storage pools are copied to secondary storage pools 470-472,called “copy storage pools.” In the illustrated embodiment, each storagepool represents a plurality of similar storage devices, such as DASDs450, optical disks 451, magnetic tape devices 452, etc. In fact, allstorage devices within a single storage pool may be identical in typeand format. Additional information about storage pools and copy pools isdisclosed in U.S. Pat. No. 6,148,412, which issued on Nov. 14, 2000 inthe names of David Maxwell Cannon et al. The entirety of the foregoingpatent is incorporated herein by reference.

Storage pools may be implemented in numerous ways, beyond that which ispracticable and necessary for discussion herein, as such would beapparent to ordinarily skilled artisans having the benefit of thisdisclosure. For instance, the primary pools 450, 451, and 452 may allshare the same copy pool. Additionally, data from one primary pool maybe backed up to multiple copy pools.

More Detail: Database

As mentioned above, the database 113 is used to store variousinformation about data contained in the storage hierarchy 114. Thisinformation, for example, includes the addresses at which data objectsare stored in the storage hierarchy 114, various characteristics of thestored data, certain client-specified data management preferences, etc.

Further explanation of the database 113 is provided below.

File Aggregation

The subsystem 102 manages various data objects, which are embodied by“managed files” for purposes of this illustration. Each managed filecomprises one user file or an aggregation of multiple constituent userfiles. The use of aggregate files is optional, however, and all managedfiles may constitute individual user files if desired. The “user” filesare created by the client stations 106, and managed by the subsystem 102as a service to the client stations 106. The subsystem 102's use ofaggregate files, however, is transparent to the client stations 106,which simply regard user files individually. This “internal” managementscheme helps to significantly reduce file management overhead costs byusing managed files constructed as aggregations of many different userfiles. In particular, the subsystem 102 treats each managed file(whether aggregate or not) as a single file during backup, move, andother subsystem operations, reducing the file management overhead tothat of a single file.

FIG. 5 shows an exemplary set of managed files 502-506. For ease ofexplanation, uppercase alphabetic designators refer to aggregate files,whereas lowercase designators point out individual user files. Thus, themanaged files 502-506 are also referenced by corresponding alphabeticdesignators A-C, for simpler representation in various tables shownbelow.

The managed file 502 includes multiple user files 502 a-502 p (alsoidentified by alphabetic designators a-p). The user files 502 a-502 pare stored adjacent to each other to conserve storage space. Theposition of each user file in the managed file 502 is denoted by acorresponding one of the “offsets” 520. In an exemplary implementation,the offsets may represent bytes of data. Thus, the first user file 502 ahas an offset of zero bytes, and the second user file 502 b has anoffset of ten bytes. In the simplified example of FIG. 5, each user fileis ten bytes long.

FIG. 5 also depicts other managed files 504, 506, each including varioususer files. In this example, the managed file 506 contains unused areas510/512 that were once occupied by user files but later deleted. Asshown in FIG. 5, the files 506 ba, 506 bh, 506 bn . . . 506 bx arepresent in the managed file 506. Additional details of file aggregationare disclosed in U.S. Pat. No. 6,098,074, which issued on Aug. 1, 2000in the names of Cannon et al. The entirety of the foregoing patent isincorporated herein by reference.

Tables

The database 113 is composed of various information including tablesthat store information about data contained in the storage hierarchy114. FIG. 1B shows the contents of the database 113 in greater detail.Namely, these tables include an inventory table 150, a storage table152, mapping tables 154, and an aggregate attributes table 156. Othertables 158 may be utilized, as well, depending upon the nature of theintended application. Each table provides a different type ofinformation, exemplified in the description below. Ordinarily skilledartisans (having the benefit of this disclosure) will quickly recognizethat the tables shown below are merely examples, that this data may beintegrated, consolidated, or otherwise reconfigured, and that theirstructure and contents may be significantly changed, all withoutdeparting from the scope of the present invention. Moreover, instead oftables, this data may be organized as one or more object-orienteddatabases, relational databases, linked lists, etc.

Inventory Table

TABLE 1, below, shows an example of the inventory table 150. Theinventory table contains information specific to each user file storedin the subsystem 102, regardless of the location and manner of storingthe user files. Generally, the inventory table cross-references eachuser file with various “client” information and various “policy”information. More particularly, each user file is listed by itsfilename, which may comprise any alphabetic, alphanumeric, numeric, orother code uniquely associated with that user file. The inventory tablecontains one row for each user file.

The client information includes information relative to the clientstation 106 with which the user file is associated. In the illustratedexample, the client information is represented by “client number”, and“source” columns. For each user file, the “client number” columnidentifies the originating client station 106. This identification mayinclude a numeric, alphabetic, alphanumeric, or other code. In thisexample, a numeric code is shown. The “source” column lists a locationin the client station 106 where the user file is stored locally by theclient. As a specific example, a user file's source may comprise adirectory in the client station.

In contrast to the client information of TABLE 1, the policy informationincludes information concerning the client's preferences for datamanagement by the subsystem 102. Optimally, this information includesthe client's preferences themselves, as well as information needed toimplement these preferences. In the illustrated example, the policyinformation is represented by the “retention time” and “active?”columns. Under the column heading “active?” the table 150 indicateswhether each user file is considered “active” or “inactive.” In oneembodiment, this is manually specified by an operator, host,application, or other source. Alternatively, the active/inactivedetermination may be made automatically, by appropriate criteria. Onesuch criterion for being “inactive” includes files that have been storedin the storage hierarchy 114 while their counterpart at a client station106 is later modified or deleted. Other examples of criteria foractive/inactive status include the frequency or recency of use of afile, the file's source location, the length of time since the file hasbeen referenced by a client station, etc. In still another embodiment,the “active?” column may be omitted, with the active/inactivedetermination being made “on the fly” whenever the active/inactivestatus of a file affects any action to be taken. The policy informationmay also include other columns (not shown), for example, listing amaximum number of backup versions to maintain, time stamps of backed-updata, etc.

TABLE 1 INVENTORY TABLE RETENTION TIME CLIENT (APPLICABLE TO FILENAMEACTIVE ? NUMBER SOURCE INACTIVE FILES) a no 1 /usr  30 DAYS b yes 1 /usr 30 DAYS c no 1 /usr  30 DAYS d yes 1 /usr  30 DAYS e no 1 /usr  30 DAYS. . . . . . 1 /usr  30 DAYS p yes 1 /usr  30 DAYS aa yes 27 D:\DATA  90DAYS ab yes 27 D:\DATA  90 DAYS ac yes 27 D:\DATA  90 DAYS ad no 27D:\DATA  90 DAYS ae yes 27 D:\DATA  90 DAYS . . . . . . 27 D:\DATA  90DAYS aj yes 27 D:\DATA  90 DAYS ba yes 3 C:\DATA 365 DAYS bh no 3C:\DATA 365 DAYS bn no 3 C:\DATA 365 DAYS . . . . . . 3 C:\DATA 365 DAYSbx yes 3 C:\DATA 365 DAYS

Storage Table

TABLE 2, below, shows an example of the storage table 152. In contrastto the inventory table 150 (described above), the storage table 152contains information about where each managed file is stored in thestorage hierarchy 114. The storage table 152 contains a single row foreach storage instance of a managed file.

In the illustrated example, the storage table 152 includes “managedfilename”, “storage pool”, “volume”, “location”, “file(s) containingsubstitute pattern,” and any other desired columns. The “managedfilename” column lists all managed files by filename. Each managed filehas a filename that comprises a unique alphabetic, alphanumeric,numeric, or other code. For each managed file, the “storage pool”identifies a subset of the storage hierarchy 114 where the managed fileresides, and more particularly, one of the primary or copy storagepools. As mentioned above, each “storage pool” is a group of storagedevices of the storage hierarchy 114 having similar performancecharacteristics. Identification of each storage pool may be made bynumeric, alphabetic, alphanumeric, or another unique code. In theillustrated example, numeric codes are used.

The “volume” column identifies a sub-part of the identified storagepool. In the data storage arts, data is commonly grouped, stored, andmanaged in logical “volumes”, where a volume may comprise a tape or aportion of a DASD. The “location” column identifies the correspondingmanaged file's location within the volume. As an example, this value maycomprise a track/sector combination (for DASDs or optical disks), atachometer reading (for magnetic tape), address, etc. The “file(s)containing substitute bit pattern” column identifies any constituentuser files of the listed managed file that have been replaced by apredetermined bit pattern rather than being physically stored.Alternatively, instead of using this column, the invention may make anonspecific notation (1) for each managed file that is a user file thatcontains the substitute bit pattern, and (2) for each managed file thatis an aggregate file having one or more constituent user files that havebeen replaced with the substitute bit pattern. In still anotherembodiment, the “file(s) containing substitute bit pattern” column maybe omitted entirely, as explained in greater detail below.

TABLE 2 STORAGE TABLE MANAGED STORAGE FILE(S) CONTAINING FILENAME POOLVOLUME LOCATION SUBSTITUTE BIT PATTERN A 1 (PRIMARY) 39 1965 A 7 (COPY)17 2378 a, c A 8 (COPY) 9 1123 a B 1 (PRIMARY) 39 4967 B 7 (COPY) 175492 ad C 1 (PRIMARY) 2 16495 C 7 (COPY) 21 439 bn

Mapping Tables

TABLES 3A-3B, below, provide an example of the mapping tables 154.Generally, these tables operate to bidirectionally cross-referencebetween aggregate files and user files. The mapping tables identify, foreach aggregate file, all constituent user files. Conversely, for eachuser file, the mapping tables identify one or more aggregate filescontaining that user file. In this respect, the specific implementationof TABLES 3A-3B includes an “aggregate→user” table (TABLE 3A) and a“user→aggregate” table (TABLE 3B).

The “aggregate→user” table contains multiple rows for each aggregatefile, each row identifying one constituent user file of that aggregatefile. Each row identifies a aggregate/user file pair by the managedfilename (“managed filename” column) and the user filename (“userfilename”).

Conversely, each row of the “user→aggregate” table lists a single userfile by its name (“user filename” column), cross-referencing this userfile to one managed file containing the user file (“managed filename”).If the user file is present in additional managed files, the mappingtables contain another row for each additional such managed file. Ineach row, identifying one user/managed file pair, the row's user file isalso cross-referenced to the user file's length (“length” column) andits offset within the aggregate file of that pair (“offset” column). Inthis example, the length and offset are given in bytes.

TABLE 3B USER -> AGGREGATE MANAGED USER (AGGREGATE) FILENAME FILENAMELENGTH OFFSET a A 10 0 b A 10 10 c A 10 20 d A 10 30 e A 10 40 . . . A10 . . . p A 10 150 aa B 10 0 ab B 10 10 ac B 10 20 ad B 10 30 ae B 1040 . . . B . . . . . . aj B 10 90 ba C 10 0 bh C 10 70 bn C 10 120 . . .C . . . . . . bx C 10 230

TABLE 3A AGGREGATE -> USER MANAGED (AGGREGATE) USER FILENAME FILENAME Aa b c d e . . . p B aa ab ac ad ae . . . aj C ba bh bn . . . bx

Aggregate Attributes Table

TABLE 4, below, shows an example of the aggregate attributes table 156.This table accounts for the fact that, after time, an aggregate file maycontain some empty space due to deletion of one or more constituent userfiles. As explained below, the subsystem 102 generally does notimmediately consolidate an aggregate file upon deletion of one or moreconstituent user files. This contributes to the efficient operation ofthe subsystem 102, by minimizing management overhead for the aggregatefiles.

If desired, to conserve storage space, reclamation may be performed toremove unused space between and within aggregate files, as taught byU.S. Pat. No. 6,021,415, which issued on Feb. 1, 2000. The reclamationprocedure, as discussed in the '415 patent, utilizes knowledge ofaggregate file attributes as maintained in the aggregate attributestable.

Each row of the aggregate attributes table represents a differentmanaged file, identified by its managed filename (“managed filename”column). Each row lists one aggregate file, along with its original sizeupon creation (“original size”), present size not including deleted userfiles (“in-use size”), and number of non-deleted user files (“in-usefiles”).

TABLE 4 AGGREGATE ATTRIBUTES TABLE MANAGED ORIGINAL IN-USE IN-USEFILENAME SIZE SIZE FILES A 160 160 16 B 100 100 10 C 240 130 13

Other Tables

The database 113 may also be implemented to include a number of othertables 158 if desired, the content and structure being apparent to thoseof ordinary skill in the art (having the benefit of this disclosure).Some or all of these tables, for instance, may be added or incorporatedinto various existing tables discussed above. In one embodiment, thedatabase 113 includes a storage pool table (not shown) indicatingwhether each storage pool is a primary or copy storage pool, rather thanincluding this information in the “storage pool” column of the storagetable 152.

Operation

Having described various structural features, an operational aspect ofthe present invention will now be described.

Signal-Bearing Media

Wherever the functionality of the invention is implemented usingmachine-executed program sequences, these sequences may be embodied invarious forms of signal-bearing media. In the context of FIG. 2, thissignal-bearing media may comprise, for example, the storage 204 oranother signal-bearing media, such as a magnetic data storage diskette300 (FIG. 3), directly or indirectly accessible by a processor 202.Whether contained in the storage 206, diskette 300, or elsewhere, theinstructions may be stored on a variety of machine-readable data storagemedia. Some examples include direct access storage (e.g., a conventional“hard drive”, redundant array of inexpensive disks (“RAID”), or anotherDASD), sequential-access storage such as magnetic tape, electronicnon-volatile memory (e.g., ROM, EPROM, or EEPROM), battery backup RAM,optical storage (e.g., CD-ROM, WORM, DVD), or other suitablesignal-bearing media including analog or digital transmission media andcommunication links and wireless communications. In an illustrativeembodiment of the invention, the machine-readable instructions maycomprise software object code, assembled from assembly language,compiled from a language such as C, etc.

Logic Circuitry

In contrast to the signal-bearing medium discussed above, some or all ofthe invention's functionality may be implemented using logic circuitry,instead of using a processor to execute instructions. Such logiccircuitry is therefore configured to perform operations to carry out themethod of the invention. The logic circuitry may be implemented usingmany different types of circuitry, as discussed above.

Backup Sequence

FIG. 6 shows a sequence 600 to back up source data to target storage,illustrating one embodiment of the present invention. For ease ofexplanation, but without any intended limitation, the example of FIG. 6is described in the context of the system 100 described above.

The routine 600 begins when the storage director 108 receives a BACKUPinstruction (step 602). This instruction may be manually sent by aclient 106, administrator 104, another process or machine, automaticallytriggered by predetermined schedule, etc. The BACKUP instructionspecifies a body of source data to back up and the target storage to beused. In the illustrated example, the source data comprises a primarystorage pool such as 450-452 (FIG. 4B), although different sizes anddefinitions of source data may be used, such as one or more volumes,physical devices, logical devices, logical surfaces, storage assemblies,extents, ranges, folders, directories, etc.

In step 604, the director 108 begins to process a first managed file ofthe source data. While this file is being processed, it is referred toas the “current” file. The director 108 may select the first and eachsubsequent managed file for processing based on any helpful set ofcriteria, such as size, order, priority, or even arbitrarily. Thestorage director 108 identifies the constituent managed files of thesource data by using the storage table 152 (TABLE 2, above).

In step 605, the storage director checks the storage table 152 todetermine whether the current file was previously backed up. If thecurrent file was previously backed up to the target storage, there is noneed to repeat another backup for this file, and step 607 advances tostep 612 (discussed below). On the other hand, if the current file hasnot been backed up, step 607 advances to step 606, discussed below.

The director 108 next determines whether the current file is anaggregate file or an individual user file (step 606). This is done byconsulting the mapping tables 154, and in particular, concluding thatthe current file is an aggregate file only if it is shown in TABLE 3A.If the current file is a user file, the director 108 determines whetherthe current file passes predetermined backup criteria (step 608), alsocalled “qualifying” for backup. In the illustrated example, user filesare only backed up if they are “active” as shown by the inventory table150 (TABLE 1, above). Alternatively, the director 108 may utilize othercriteria to determine whether files qualify for backup, which may beavailable by predetermined list (to minimize overhead) as withactive/inactive status, or this determination may be made “on the fly”by examining relevant characteristics of the current file such as size,priority, age, content, owner, etc.

Instead of being written to storage, inactive files are replaced with aprescribed dummy pattern, as explained below. More particularly, step608 advances to step 609 if the current file passes the backup criteria(i.e., is active), in which case the storage director 108 writes thefile to target storage. Otherwise, if the current file fails the backupcriteria (i.e., is inactive), the storage director 108 writes aprescribed bit pattern to target storage instead of the current userfile (step 610). The length of the bit pattern need not match that ofthe replaced user file, since individual user files have no offsets tobe preserved (unlike aggregate files as discussed below). Uponcompletion of either step 609 or 610, the storage director 108 insertsan entry into the storage table 152 to show that the current file hasbeen backed up; if step 610 was executed, the “file(s) containingsubstitute bit pattern” column of this same entry reflects that the bitpattern has been used in substitution for the current user file.

In an exemplary embodiment, the predetermined bit pattern may beprestored in a memory buffer (not shown) of the subsystem 102 in orderto expedite repeated copying of the bit pattern to target storage. Thebit pattern of step 610 (and step 622, below) is selected to be easilyrecognized and efficiently compressed by automatic software and/orhardware compression processes that physically write data to the storagehierarchy 114. More particularly, the bit pattern is selected such that,if provided as input to a certain digital data compression process, itwould be compressed with at least a particular predicted “compressionefficiency”. Compression efficiency may be measured for example, as aratio of pre-compression to post-compression storage size, or anothersuitable computation. The compression efficiency is “predicted” basedupon knowledge of how the implemented compression process treats thepredetermined bit pattern; this may be near or even equal the “actual”compression efficiency achieved when compression is subsequentlyperformed. Optimally, the predetermined bit pattern is selected becauseof its high compressibility, thereby achieving the maximum compressionefficiency when compressed. In this respect, certain bit patterns may bechosen because they have an obviously high compressibility by manycompression processes, without requiring any specific knowledge ofparticular compression processes operation. As an example, desirable bitpatterns include a sequence of repeating binary zeros, or a sequence ofrepeating binary ones. Both of these patterns are easily compressed bymost known compression processes such as the well known Lempel-Ziv-Welch(LZW) and run length encoding (RLL) techniques. Preferably, the same bitpattern is used each time step 610 (and step 622, below) is invoked,although the bit pattern may be varied between and/or within steps 610,622 if desired.

In step 612, the storage director 108 asks whether the source dataincludes any remaining managed files to process. If so, processing ofthe next managed file begins (step 614), with this file becoming thecurrent file for processing starting in step 605. Otherwise, if step 612finds that the source data does not contain any other managed files toprocess, the program 600 ends (step 616).

In contrast to the foregoing description of processing individual userfiles, a different sequence is used if step 606 finds that the currentfile is an aggregate file. Namely, step 606 advances to step 618, wherethe storage director 108 begins by considering a first user file withinthe subject aggregate file. While this constituent user file is beingprocessed, it is referred to as the “current” user file. The director108 may select the first and each subsequent constituent user file forprocessing based on any helpful criteria, such as size, order, priority,or even arbitrarily. The storage director 108 identifies the constituentuser files within the current aggregate file by using the mapping tables154 (namely, TABLE 3A shown above).

The director 108 next determines whether the current user file passesthe predetermined backup criteria (step 620), namely, whether thecurrent user file is an “active” file. If the current file passes thebackup criteria (i.e., is active), the storage director 108 writes thefile to target storage (step 621). Otherwise, if the current file failsthe backup criteria (i.e., is inactive), the storage director 108 writesthe prescribed bit pattern to target storage instead of the current userfile (step 622). In the illustrated example, length of the prescribed,substitute bit pattern (uncompressed) is the same as the length of thecurrent user file (uncompressed) in order to preserve the originaloffsets of user files within the subject aggregate file.

In step 624, the storage director 108 asks whether the current aggregatefile includes any other constituent user files to process. If so,processing of the next user file begins (step 626), with this filebecoming the current user file for processing starting in step 620.Otherwise, if step 624 finds that the current aggregate file does notcontain any other user files to process, then processing of the currentaggregate file is complete. At this point (step 625), the storagedirector 108 inserts an entry into the storage table 152 to show thatthe current aggregate file has been backed up, and if appropriate, whichuser files of that aggregate file contain the predetermined bit pattern.

After step 625, step 628 asks whether the source data contains any moremanaged files left to process. If so, the next managed file is selected(step 614) and processing of that file begins in step 605. Otherwise,the program 600 ends in step 630.

Sequence for Reclamation

The operations of sequence 600 may be applied to a backup data operation(as discussed above), or to achieve “reclamation” of backup data inorder to conserve space.

Broadly, reclamation consolidates data storage space by eliminatingunwanted or unused space. More specifically, in the context of theillustrated environment, reclamation involves applying the steps 600 toform a further copy of backup data, with any user files that have becomeinactive since being backed up being replaced with the substitute bitpattern. In reclamation, however, steps 605-607 are omitted because thedata inherently exists. Also, in steps 609, 610, 625, metadata isadditionally updated to remove references to the original copy.

Sequence for Restore to Primary Storage Pool

FIG. 7 shows a sequence 700 to restore data from one or more copystorage pools to one or more primary storage pools in the hierarchy 114due to the data of a primary storage pool being lost or inaccessible.For ease of explanation, but without any intended limitation, theexample of FIG. 7 is described in the context of the system 100described above.

The routine 700 begins when the storage director 108 receives a RESTOREinstruction (step 702). As one example, this instruction may be manuallyinstituted by the administrator 104. Alternatively, the instruction mayemanate from another process or machine, automatic trigger,predetermined schedule, etc. The RESTORE instruction identifies subjectfiles and a primary storage pool, providing directions to restore thesefiles by copying them from one or more copy storage pools back into theprimary storage pool. The RESTORE instruction identifies the subjectfiles by storage location, name, rules, characteristics, wildcardcharacters, or any other criteria useful in determining which filesshould be restored.

After step 702, the storage director 108 begins to process the managedfiles identified in the RESTORE instruction of step 702, one managedfile at a time. More particularly, in step 704, the storage director 108starts with a first managed file of the data to be restored. While thisfile is being processed, it is referred to as the “current” file. Thedirector 108 may select the first and each subsequent managed file forprocessing based on any helpful criteria, such as size, order, priority,efficiency, or even arbitrarily.

After step 704, the storage director 108 asks whether the currentmanaged file has been previously backed up, and identifies eachdifferent backup if there are more than one (step 712). If the currentmanaged file is a user file, then step 712 involves consulting thestorage table 152 (TABLE 2, above) to determine whether this file existsin any copy storage pool, and to identify these copy storage pools (ifany). If step 712 finds that there no backups, step 712 advances to step716, which fails the RESTORE operation for this file and returns asuitable error code or returns a suitable error message to the source ofthe original RESTORE instruction.

If step 712 finds one or more backup copies of the current file, thedirector 108 chooses an appropriate backup site from which to carry outthe restoration (step 714). The choice of step 714 may be based uponvarious considerations, such as the following:

-   -   1) if backup is on tape, choosing a backup volume that is        already mounted to tape accessing equipment.    -   2) choosing a backup volume that is not being used by another        process.    -   3) if backup is on tape, choosing a backup volume that is in an        automated tape library rather than requiring manual mounting or        delivery from an off site location.    -   4) administrator preference.    -   5) choosing the backup site likely to provide fastest read time        based on device performance attributes.    -   6) if the current file is an aggregate file, choosing a backup        site where a minimum number of constituent user files of the        aggregate file have been replaced with the substitute bit        pattern. This is determined by consulting the storage table 152        to determine which constituent user files of the current        (aggregate) file contain valid data and which (if any) contain        the substitute bit pattern. In the illustrated embodiment, the        “file(s) contain substitute bit pattern” column of the storage        table 152 indicates whether each constituent user file contains        valid data or not. In a different implementation, where the        “file(s) contain substitute bit pattern” column generally        indicates whether any constituent user file contains the        substitute bit pattern (without identifying which user file),        then the storage director additionally consults the mapping        tables 154 (namely, TABLE 3A) and the inventory table 150 to        determine which constituent user files are “active” and which        are “inactive.” As for the backed up user files shown to be        inactive, these are assumed to be replaced with the substitute        bit pattern; backed up user files shown to be active are assumed        to represent valid data. For this condition to hold, and in the        particular embodiment where the “file(s) contain substitute bit        pattern” column does not specifically identify user files        containing the substitute bit pattern, the storage director 108        necessarily manages the inventory table 150 so as to prevent        inactive files from ever becoming active; to allow flip-flopping        would possibly permit restoration of null data. Alternatively,        in embodiments where the “file(s) contain substitute bit        pattern” column is omitted from the storage table 152, the        backup data itself may be examined by comparing at least part of        each constituent user file to the substitute bit pattern to        determine whether useful data is represented therein.    -   If all constituent user files of the current aggregate file have        been substituted, then this data is not usable for the present        RESTORE operation 10 and step 714 jumps to step 716. Here, the        storage director 108 takes action to invalidate all user files        by removing each user file's data from the inventory table 150,        removing references to the aggregate and its constituent user        files from the mapping tables and the aggregate attributes        table, removing all entries related to this aggregate file from        the storage table, and reporting failure of the RESTORE        operation for the current file.    -   7) if the current file is a user file rather than an aggregate        file, step 714 chooses a backup site where the user file has not        been replaced with the substitute bit pattern. This is done by        consulting the “file(s) contain substitute bit pattern” column        of the storage table 152. Alternatively, in embodiments where        this column is not used, the backup data itself may be examined        by comparing at least a part of the data to the substitute bit        pattern to determine whether useful data is represented therein.        If the current user file has no backups other than those with        the substitute bit pattern, then this data is not usable for the        present RESTORE operation and step 714 jumps to step 716 to fail        the RESTORE operation. In this case, the data is effectively        useless and the storage director 108 takes action to invalidate        the user file, whereby the storage director 108 removes the user        file's data from the inventory table 150, removes storage table        entries for the current user file, and reports failure of the        RESTORE operation for this user file.

After step 714, the storage director 108 carries out the restoreoperation from the chosen backup site (step 710). Additionally, step 710updates the storage table 152 in order to reference the location of therestored file and to delete the reference to the file's originallocation. Namely, the backup data is copied to the primary storage poolidentified in the original RESTORE instruction. After step 710 (or step716, discussed above), the director 108 asks whether there are moremanaged files left to restore, according to the original RESTOREinstruction (step 718). If so, the director 108 proceeds to the nextmanaged file (step 708), and returns to step 712. Otherwise, if thereare no further managed files to restore, step 718 advances to step 720,where the routine 700 ends.

Sequence For Restore to Client Station

FIG. 8 shows a sequence 800 to restore data from the storage hierarchy114 to a client station 106 due to that data becoming lost, deleted, orinaccessible at that client station 106. For ease of explanation, butwithout any intended limitation, the example of FIG. 8 is described inthe context of the system 100 described above.

The routine 800 begins when the storage director 108 receives a RESTOREinstruction (step 802). This instruction may be manually orautomatically submitted by or on behalf of a client station 106, such asthe client station 106 that has experienced the data loss. The RESTOREinstruction identifies one or more user files to be restored by sendingthem from one or more primary or copy storage pools back to the clientstation 106. The RESTORE instruction identifies the subject user filesby name, rules, characteristics, wildcard characters, or any othercriteria useful in determining which files should be restored.

After step 802, the storage director 108 begins to process a first oneof the user files identified in the RESTORE instruction (step 804).While this file is being processed, it is referred to as the “current”file. The director 108 may select the first and subsequent user filesfor processing based on any helpful criteria, such as size, order,priority, efficiency, or even arbitrarily.

After step 804, the storage director 108 attempts to locate the currentuser file in its primary storage location (step 806). In the illustratedenvironment, this step is performed using the mapping tables 154(namely, TABLE 3B, above) and the storage table 152 (TABLE 2, above). Ifthe current user file was found, the storage director 108 reads thecurrent user file from the primary location and copies it to the clientstation 108 (also in step 806). After step 806, step 808 asks whetherthe operation of step 806 succeeded. If so, step 808 advances to step816, described below.

On the other hand, if the current user file cannot be found at theprimary location, step 808 advances to step 810, which asks whether thecurrent user file had been previously backed up from the primarylocation, and identifies each different backup if there are more thanone. This is performed by consulting the storage table 152 (TABLE 2,above) to determine whether this file exists in any copy storage pool,and to identify these copy storage pools (if any). If step 810 findsthat there no backups, step 810 advances to step 814, which fails theRESTORE operation for this user file and returns a suitable error codeor returns a suitable error message to the source of the originalRESTORE instruction.

If the current user file has been previously backed up, the director 108chooses an appropriate backup site (step 812). If there are multiplebackup sites, the choice among backup sites may be made using similarconsiderations as discussed above in conjunction with FIG. 7. If thereare one or more backups available, but the current user file has beenreplaced by the substitute bit pattern in each backup site, then step812 jumps to step 814 where the storage director fails the RESTOREoperation. In this case, the storage director 108 may optionallyinvalidate the file in the manner discussed above. After step 814,control passes to step 816 to determine whether there are more filesleft to restore. If step 812 completes successfully, however, thestorage director 108 proceeds to step 815, where it carries out therestore operation from the chosen backup site.

After step 815 (or step 814 or an affirmative answer to step 808), step816 checks whether there are more user files left to restore, accordingto the original RESTORE instruction of step 802. If so, the director 108proceeds selects the next user file in step 818, and returns to step806. Otherwise, if there are no further user files to restore, step 816advances to step 820, where the routine 800 ends.

Other Embodiments

While the foregoing disclosure shows a number of illustrativeembodiments of the invention, it will be apparent to those skilled inthe art that various changes and modifications can be made hereinwithout departing from the scope of the invention as defined by theappended claims. Furthermore, although elements of the invention may bedescribed or claimed in the singular, the plural is contemplated unlesslimitation to the singular is explicitly stated. Additionally,ordinarily skilled artisans will recognize that operational sequencesmust be set forth in some specific order for the purpose of explanationand claiming, but the present invention contemplates various changesbeyond such specific order.

1.-17. (canceled)
 18. A method of backing up files to storage,comprising operations of: determining whether a file qualifies to bebacked up to a storage; writing the file to the storage in response todetermining that the file qualifies to be backed up; and writing a dummypattern to the storage, wherein the dummy pattern is unrelated to dataof the file, and wherein the dummy pattern is written for multiple filesthat do not qualify to be backed up and replaces the file data in thestorage.
 19. The method of claim 18, wherein determining whether thefile qualifies to be backed up comprises determining whether the filehas an active or inactive status, wherein the file having the activestatus qualifies to be backed up and the file having the inactive statusis replaced with the dummy pattern in the storage.
 20. The method ofclaim 19, wherein the inactive or active status for files is manuallyset by an operator, host or application.
 21. The method of claim 18,wherein the file is also stored in a client storage, wherein determiningwhether the file qualifies to be backed up to the storage comprisesdetermining whether the file is stored in a storage hierarchy and acounterpart of the file is stored in the client storage and is latermodified or deleted at the client storage.
 22. The method of claim 18,wherein client computers are connected to a storage subsystem over anetwork, and wherein the client computers create the files that aremanaged by the storage subsystem as a service to the client computers.23. The method of claim 22, wherein the storage subsystem includes astorage director, further comprising: receiving, by the storagedirector, a backup instruction, from one of the clients over thenetwork, specifying source files to back up at the storage, wherein thestorage director performs the operations of the determining whether thefile qualifies to be backed, the writing the file and the writing thedummy pattern for each of the specified source files in response toreceiving the backup instruction.
 24. The method of claim 18, whereinthe file comprises an aggregate file comprised of multiple constituentfiles, wherein the operations of the determining whether the filequalifies to be backed, the writing the file and the writing the dummypattern are performed for each of the constituent files, and wherein thedummy pattern has a same length as the constituent file to preserveroriginal offsets of the constituent files in the aggregate file.
 25. Astorage media including a program of machine-readable instructionsexecutable by a processor to perform operations to back up files tostorage, the operations comprising: determining whether a file qualifiesto be backed up to the storage; writing the file to the storage inresponse to determining that the file qualifies to be backed up; andwriting a dummy pattern to the storage, wherein the dummy pattern isunrelated to data of the file, and wherein the dummy pattern is writtenfor multiple files that do not qualify to be backed up and replaces thefile data in the storage.
 26. The storage media of claim 25, whereindetermining whether the file qualifies to be backed up comprisesdetermining whether the file has an active or inactive status, whereinthe file having the active status qualifies to be backed up and the filehaving the inactive status is replaced with the dummy pattern in thestorage.
 27. The storage media of claim 26, wherein the inactive oractive status for files is manually set by an operator, host orapplication.
 28. The storage media of claim 25, wherein the file is alsostored in a client storage, wherein determining whether the filequalifies to be backed up to the storage comprises determining whetherthe file is stored in a storage hierarchy and a counterpart of the fileis stored in the client storage and is later modified or deleted at theclient storage.
 29. The storage media of claim 25, wherein the storagemedia and the processor are included in a storage subsystem incommunication with client computers over a network, wherein the clientcomputers create the files that are managed by the storage subsystem,and wherein the storage subsystem manages the files as a service to theclient computers.
 30. The storage media of claim 29, wherein theoperations further comprise: receiving a backup instruction, from one ofthe client computers over the network, specifying source files to backup at the storage, wherein the storage subsystem performs the operationsof the determining whether the file qualifies to be backed, the writingthe file and the writing the dummy pattern for each of the specifiedsource files in response to receiving the backup instruction.
 31. Thestorage media of claim 25, wherein the file comprises an aggregate filecomprised of multiple constituent files, wherein the operations of thedetermining whether the file qualifies to be backed, the writing thefile and the writing the dummy pattern are performed for each of theconstituent files, and wherein the dummy pattern has a same length asthe constituent file to preserver original offsets of the constituentfiles in the aggregate file.
 32. A storage system, comprising: storagefor storing files; a storage director, programmed to perform operationsfor backing up files to the storage, the operations comprising:determining whether a file qualifies to be backed up to the storage;writing the file to the storage in response to determining that the filequalifies to be backed up; and writing a dummy pattern to the storage,wherein the dummy pattern is unrelated to data of the file, and whereinthe dummy pattern is written for multiple files that do not qualify tobe backed up and replaces the file data in the storage.
 33. The storagesystem of claim 32, wherein determining whether the file qualifies to bebacked up comprises determining whether the file has an active orinactive status, wherein the file having the active status qualifies tobe backed up and the file having the inactive status is replaced withthe dummy pattern in the storage.
 34. The storage system of claim 33,wherein the inactive or active status for files is manually set by anoperator, host or application.
 35. The storage system of claim 32,wherein the storage system is in communication with a plurality ofclients over a network, wherein the file is also stored in one of theclients, and wherein determining whether the file qualifies to be backedup to the storage comprises determining whether the file is stored in astorage hierarchy in the storage and a counterpart of the file is storedin the client and is later modified or deleted at the client.
 36. Thestorage system of claim 32, wherein the storage system is incommunication with a plurality of clients over a network, wherein theclients create the files that are managed by the storage director, andwherein the storage director manages the files as a service to theclients.
 37. The storage system of claim 36, wherein the operationsfurther comprise: receiving a backup instruction, from one of theclients over the network, specifying source files to back up at thestorage, wherein the storage director performs the operations of thedetermining whether the file qualifies to be backed, the writing thefile and the writing the dummy pattern for each of the specified sourcefiles in response to receiving the backup instruction.
 38. The method ofclaim 32, wherein the file comprises an aggregate file comprised ofmultiple constituent files, wherein the operations of the determiningwhether the file qualifies to be backed, the writing the file and thewriting the dummy pattern are performed for each of the constituentfiles, and wherein the dummy pattern has a same length as theconstituent file to preserver original offsets of the constituent filesin the aggregate file.